Driving circuit for brushless DC motor

ABSTRACT

A driving circuit for a brushless DC motor provided with an element for detecting the angular position of the rotor of the motor, characterized in that by the output of the differential amplifier for amplifying the output of the detecting element, a switching element is rendered on and off during a certain portion of the duration of the output of the detecting element and that the motor is rotated at a constant speed by applying a rotation control voltage to an element for energizing the stator winding of the motor during the non-conductive period.

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

This invention relates to a driving circuit for brushless DC motor furnished with an angular position detecting element.

2. DESCRIPTION OF THE PRIOR ART

Hitherto, there is known a driving circuit for a motor, in which the angular positions of the rotor of the motor are detected by means of a contactless detector so as to control the currents through the corresponding coils of the armature winding in response to the output of the detector.

With the conventional driving circuit of this kind, the corresponding coils of the stator winding are energized with the signal obtained by converting the output of the angular position detecting elements provided along the stator into a signal having a constant pulse width suitable for motor-driving by the use of a pulse-forming circuit including a level shifting circuit comprising zener diodes.

However, such a conventional driving circuit requires many circuit elements such as zener diodes and resistors so that the overall circuit becomes complicated and also consumes a considerable power. This gives rise to a drawback that the integration of the driving circuit is very difficult. Further, the conventional driving circuit is liable to be affected by the fluctuation of the ambient temperature so that it cannot be free from a drawback that the rotational speed adversely fluctuates.

SUMMARY OF THE INVENTION

The main object of this invention is to provide a driving circuit for a brushless DC motor, which consumes a small power, is simple in circuit configuration and can easily be integrated.

Another object of this invention is to provide a driving circuit for a brushless DC motor, which can stably operate in spite of the change in the ambient temperature, with a high reliability.

According to the feature of this invention, which has been made to attain these objects, the AC signal voltage delivered from the angular position detecting element is amplified through a differential amplifier and then converted to an AC signal current; the current as the difference between the AC signal current and the current supplied from a constant current source, is taken out to determine a desired conduction angle in accordance with the product of the difference current and the speed control signal; and the motor is rotated at a constant speed by energizing the coils of the stator winding in accordance with the desired conduction angle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional driving circuit.

FIG. 2 a driving circuit as an embodiment of this invention.

FIGS. 3 to 6 show the waveforms of signals appearing at several points in the circuit shown in FIG. 2, useful for explaining the operation of the circuit. FIG. 7 is an electric circuit as the detail of a portion of the circuit shown in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 showing a conventional driving circuit for a brushless DC motor, reference numeral 1 indicates a rotor of permanent magnet having a pair of N and S poles; 2 and 3 Hall elements for detecting the angular positions of rotation of the rotor 1; 4 to 7 coils constituting a rotor winding; 8 and 9 differential amplifiers; 10 and 11 pulse-forming circuits each including, for example, a level shift circuit; 12 a detecting circuit for obtaining a voltage whose amplitude is proportional to the rotational speed of the motor; 13 a speed control circuit; 14 a switching circuit; 15 to 18 transistors for driving the coils of the stator winding; and 19 a DC power source.

The brushless DC motor mainly comprises a rotor 1, coils 4 to 7 of the stator winding and Hall elements 2 and 3 provided in the vicinity thereof.

As seen in FIG. 1, the Hall elements 2 and 3 are so connected as to drive the corresponding coils 4 to 7 of the stator. Namely, the signals induced in the Hall elements 2 and 3 by the rotating magnetic field are amplified by the differential amplifiers 8 and 9 and further converted to pulse signals having constant pulse widths suitable for motor driving by the pulse-forming circuits 10 and 11. Simultaneously, a voltage having an amplitude proportional to rotational speed is taken out from the terminals of the stator winding by means of the detecting circuit 12 and the voltage, i.e. the output of the detecting circuit 12, is applied through the speed control circuit 13, to the switching circuit 14 which is turned on and off by the pulse signals.

The outputs of the switching circuit 14 are applied to the bases of the transistors 15 - 18 for driving the corresponding coils of the stator so that the motor is rotated at a constant speed by controlling the currents flowing through the transistors.

A level shift circuit consisting of zener diodes etc. is used in the pulse-forming circuit having a function of determining a conduction angle. The conduction angle mentioned here refers to the duration of the current flowing through the stator winding, expressed in terms of angle.

The conventional circuit described above is complicated, especially in its driving circuit, and encounters problems on the simplification of circuit and the reduction of consumed power. Further, the conventional circuit has a drawback that the rotational characteristic is degraded due to the variation of the conduction angle, caused by the fluctuation of temperature in the detecting element.

FIG. 2 shows a driving circuit as an embodiment of this invention, in which a reference numeral 2 indicates a silicon Hall element serving as an angular position detecting element; 5 and 7 the coils of a stator winding; 19 a DC power source; 20 a counter electromoting force detecting circuit; 21 a speed control circuit; reference characters T₁ - T₁₆ designate transistors; D₁ - D₄ diodes; R₁ - R₁₃ fixed resistors; C₁ and C₂ capacitors; and R₂₀ a variable resistor.

In FIG. 2, for simplicity's sake, is shown a driving circuit for a two-phase, two-pole, brushless motor. For a multi-phase, multi-pole motor, circuits each including Hall element have only to be incorporated according to the structure of the motor.

The output terminals of the Hall element 2, which generates AC voltages V₁ and V₂ by the magnetic field of the rotor of the motor, are connected with with the bases of the transistors T₂ and T₆ constituting a differential amplifier. Transistors T₁, T₅, T₇ and T₁₀ constitute a constant current circuit for supplying a desired constant current. One ends of the load resistors R₂ and R₄ are connected with the collectors of the transistors T₂ and T₆ while the other ends of the resistors R₂ and R₄ are connected in common with the power source 19 through the diodes D₁ and D₂ for compensating the variation of the conduction angle due to the thermal fluctuation of the angular position detecting signal. The collector of the transistor T₁ is connected with the power source 19 via the resistor R₁ and the collector of the transistor T₅ is connected with both the emitters of the transistors T₂ and T.sub. 6.

A set of transistors T₃ and T₄ for current level shifting are coupled to the collector of the transistor T₂ while a set of transistors T₇ and T₉ for current level shifting are coupled to the collector of the transistor T₆, and the emitters of the transistors T₄ and T₉ are connected respectively with the collectors of the transistors T₈ and T₁₀ so as to take out the above-mentioned difference current for determining the conduction angle. The set of transistors T₃ and T₄ and the set of the transistors T₇ and T₉ are each in the Darlington configuration.

The emitters of the transistors T₄ and T₉ are connected respectively with the transistors T₁₁ and T₁₄ serving as switching elements. One ends of the coils 5 and 7 of the stator winding are connected with the power source 19 and the other ends of the coils are connected with the counter e.m.f. detecting circuit 20 via the diodes D₃ and D₄. The output of the detecting circuit 20 is connected with the speed control circuit 21 via the resistors R₁₁ and R₂₀. The output of the speed control circuit 21 is connected with the collectors of the transistors T₁₁ and T₁₄ via the resistors R₇ and R₉, respectively. The collectors of the transistors T₁₁ and T₁₄ are grounded through the resistors R₆ and R₈ and also connected with the bases of the transistors T₁₂ and T₁₅ via the resistors R₁₂ and R₁₃. The collectors and the emitters of the transistors T₁₂ and T₁₅ are connected respectively with the collectors and the bases of the transistors T₁₃ and T₁₆ for driving the coils 5 and 7 of the stator winding. The collectors of the transistors T₁₃ and T₁₆ are connected respectively with the other ends of the coils 5 and 7 of the stator winding and also grounded through the capacitors C₁ and C₂. The emitters of the transistors T₁, T₅, T₈, T₁₀, T₁₁, T₁₃, T₁₄ and T₁₆ are grounded and the collectors of the transistors T₄ and T₉ are connected with the power source 19 respectively via the resistors R₃ and R₅ and the voltage of the DC power source 19 is applied to the Hall element 2 through the resistor R₁₀.

Now, the operation of the circuit shown in FIG. 2 will be described with the aid of FIG. 4 showing the waveforms appearing at several points in the circuit in FIG. 2.

When the motor is rotating, the voltages V₁ and V₂ having sinusoidal waveforms as shown in FIG. 3 are derived at the output terminals of the Hall element 2 due to the rotating magnetic field of the rotor. The voltages V₁ and V₂ are amplified by the differential amplifier constituted of the transistors T₂ and T₆ and then they are converted, as described before, to the currents I₁ ' and I₂ ' as shown in FIG. 4. The currents I₁ ' and I₂ ' are further divided respectively into the currents I₁ and I_(s), I₁ flowing into the base of the transistor T₁₄ and I_(s) flowing through the transistor T₈, and into the currents I₂ and I_(s), I₂ flowing into the base of the transistor T₁₁ and I_(s) through the transistor T₁₀. The currents I₁ and I₂, flowing into the bases of the transistors T₁₄ and T₁₁, serve to determine the conduction angle and can be approximated by the expressions given below. ##EQU1## , where E_(D) is the forward voltage drop across the diodes D₁ and D₂, E_(B) the forward voltage drop between the base and the emitter of the transistors T₃ or T₇, I_(o) the preset current value in the differential amplifier, I_(s) the constant current for determining the conduction angles, and e_(H) sin ωt the voltages as the amplified versions of the outputs of the Hall element, appearing across the load resistors R₂ and R₄ of the differential amplifier.

Namely, the currents I₁ and I₂ flowing into the bases of the transistors T₁₄ and T₁₁ are changed depending upon the voltage delivered from the Hall element, and converted to the waveforms shown in FIG. 5. If it is assumed that I_(B) is the base current at which the transistors T₁₁ and T₁₄ are turned on and that V₃ and V₄ are the components of the control voltage V divided by the resistors R₆ and R₇ and by the resistors R₈ and R₉, as shown in FIG. 2, then the voltages V₃ and V₄ take the waveforms as shown in FIG. 6 in the region where I₁ or I₂ is greater or smaller than I_(B).

Since the transistors T₁₄ and T₁₁ are both conductive in the region where both I₁ and I₂ exceed I_(B), the voltages V₃ and V₄ varnish. Accordingly, the transistors T₁₆ and T₁₃ for driving the coils of the stator winding are cut off so that the currents through the coils are interrupted. The transistor T₁₄ (T₁₁) is cut off during the period T for which I₁ (I₂) is smaller than I_(B).

The counter electromotive forces induced in the coils 5 and 7 of the stator winding, which are proportional to the rotational speed of the motor, are sent through the diodes D₄ and D₃ to the counter e.m.f. detecting element 20. After the output of the detector 20 has been divided by the circuit including the resistor R₂₀, a speed control voltage is generated by the speed control circuit 21. The control voltage is further divided by the resistors R₆ and R₇ and by the resistors R₈ and R₉ and the respectively divided voltages V₃ and V₄ are applied to the collectors of the transistors T₁₁ and T₁₄. In this case, the transistors T₁₁ and T₁₄ constitute a multiplying circuit so that when the transistors T₁₁ and T₁₄ are non-conductive, i.e. in the state 0, the result of multiplication is 0 while when they are conductive, i.e. in the state 1, the result of multiplication is V₃ and V₄. Therefore, when the transistor T₁₁ (T₁₄) is cut off, the voltage V₃ (V₄) is applied through the resistor R₁₂ (R₁₃) to the base of the transistor T₁₂ (T₁₅) so that the transistor T₁₃ (T₁₆) for driving the coil 5 (7) of the stator winding is brought into conduction. The collector-emitter voltage of the transistor T₁₃ (T₁₆) is so controlled as to cause a desired current to flow through the coil 5 (7) of the stator winding so that the motor may be rotated by the desired current.

If the speed control circuit has only to be so designed as to deliver a speed control voltage V which is low (or high) when the rotational speed of the motor is high (or low), the currents flowing through the transistors T₁₆ and T₁₃ can be controlled by the voltage V so that the motor may be rotated at a constant speed.

FIG. 7 shows in detail examples of the counter e.m.f. detecting circuit 20 and the speed control circuit 21, shown in FIG. 2, and in this figure are shown a differential amplifier A, resistors R₁₄ - R₁₇ and a capacitor C₃.

The resistor R₁₄ together with the variable resistor R₂₀, serves to divide the voltage supplied through the diodes D₃ and D₄ while the resistors R₁₅ and R₁₆ serve to divide the voltage of the DC power source 19 and to apply the divided voltage to the positive input terminal + of the differential amplifier A. The difference amplifier A and the resistor R₁₇ and the capacitor C₃ constitute an integrator.

As seen in FIG. 7, the counter electromotive forces proportional to the rotational speed are taken out of the coils 5 and 7 of the stator winding through the diodes D₄ and D₃ and the counter electromotive forces are each divided by the resistor R₁₄ and the variable resistor R₂₀ and the divided component is applied to the negative input terminal - of the differential amplifier A. On the other hand, the voltage of the power source is divided by the resistors R₁₅ and R₁₆ and the divided component is applied to the positive input terminal + of the differential amplifier A so as to obtain the speed control voltage V therefrom.

The fluctuation of the rotational speed due to the thermal fluctuation in the Hall elements 2 and 4 for detecting the angular positions, can be compensated and therefore reduced to a considerable extent by means of the diodes D₁ and D₂, as shown in FIG. 2. Namely, as apparent from I₁ and I₂ described above and from FIGS. 4 and 5, the AC components in I₁ and I₂ decrease with the increase in the temperature of the angular position detecting element if the output of the detecting element has a negative temperature coefficient. Correspondingly, however, the forward voltage drops across the diodes D₁ and D₂ decrease so that the DC components of I₁ and I₂ also decrease. Thus, with suitable circuit constants, the variation in the output of the angular position detecting element due to the fluctuation of temperature can be cancelled and the conduction angle can be kept fixed. It is also easy to set the conduction angle at any desired value.

Further, even if due to the change in, for example, the ambient temperature the voltage of the power source 19 is changed to change the value of the current supplied from the constant current circuit constituted of the transistors T₁, T₅, T₈ and T₁₀, the output of the differential amplifier consisting of the transistors T₂ and T₆ accordingly changes to cancel the resultant change so that a stable operation can be assured.

Moreover, since the difference current, obtained by the constant current circuit consisting of transistors, is applied directly to the conduction angle determining circuit, the overall circuit can be simplified.

Furthermore, since the operating current for the driving circuit can be made small by choosing the preset current of the constant current circuit to be a small value, the power to be consumed can considerably be decreased. Consequently, a driving circuit well adapted for integration, having a high reliability can be obtained according to this invention. 

We claim:
 1. A driving circuit for a brushless DC motor consisting mainly of a rotor of permanent magnet having a pair of N and S poles, a stator winding having a plurality of coils and at least one detecting element for detecting the angular position of said rotor; said driving circuit comprisinga first means for amplifying the AC voltage derived from said detecting element and for converting the amplified voltage to an AC current; a second means for supplying a predetermined constant current; a third means for obtaining a current as the difference of the current from said first means and said constant current from said second means; a fourth means for generating a control signal corresponding to a desired speed; a fifth means for determining the conduction angle in accordance with the difference current derived from said third means and said control signal derived from said fourth means; and a sixth means for controlling the currents flowing through said coils of said stator winding in accordance with said conduction angle determined by said fifth means.
 2. A driving circuit as claimed in claim 1, wherein said detecting element is a Hall element.
 3. A driving circuit as claimed in claim 1, wherein said first means includes a differential amplifier for amplifying the voltage between the terminal of said detecting element.
 4. A driving circuit as claimed in claim 1, wherein said third means is a constant voltage circuit constituted of transistors.
 5. A driving circuit as claimed in claim 1, wherein said fourth means includes detecting means for detecting the counter electromotive forces induced in said coils of said stator winding and mans for generating control signals in accordance with the outputs of said detecting means.
 6. A driving circuit as claimed in claim 1, wherein said sixth means includes a switching element which is turned on and off in accordance with said conduction angle.
 7. A driving circuit as claimed in claim 1, wherein said fifth means is constituted of a switching element which is turned on and off in response to said difference current derived from said third means and an output means for delivering a voltage in accordance with said control signal in one of the on- and off-states of said switching element.
 8. A driving circuit as claimed in claim 1, wherein said first means includes a compensating element for compensating the fluctuation of the output of said detecting element due to the variation in temperature.
 9. A driving circuit as claimed in claim 3, wherein said differential amplifier comprises a first and a second transistors having their input electrodes connected respectively with the output terminals of said detecting element, a first constant current circuit connected in common with the first output electrodes of said first and second transistors, a first and second transistors, a first and a second load resistors having their one ends connected respectively with the second output electrodes of said first and second transistors and a temperature compensating element connected in common with the other ends of said first and second load resistors.
 10. A driving circuit as claimed in claim 9, wherein said temperature compensating element comprises at least one diode.
 11. A driving circuit as claimed in claim 9, wherein said first means includes a first and a second transistor circuits for respectively converting the voltages derived from said second output electrodes of said first and second transistors to the corresponding currents.
 12. A driving circuit as claimed in claim 1, wherein said third means includes a second and a third constant current circuits each consisting of transistors connected with said first and second transistor circuits.
 13. A driving circuit as claimed in claim 5, wherein said control signal generating means includes an integrator for generating an output in accordance with the output of said detecting means. 